In gas turbine engines a wake is produced downstream of components past which gases flow, and especially components which turn the direction of the gas flow. This wake may roll up into a discrete vortex of intense vorticity that can result in significant flow distortions as well as mixing and interference losses downstream thereof. Such components include turbine aerofoils, and the wake produced downstream thereof can have an effect upon subsequent blade rows where it results in flow distortions, incidence and other loss mechanisms.